1. Field
Disclosed herein is a method for producing anisotropic magnetic powder and/or a bonded anisotropic magnet produced from such a powder.
2. Description of Related Art
In the production of Nd—Fe—B sintered magnets, sintered magnetic residues, also known as magnetic scrap metal, are formed. This magnetic scrap metal is composed, for example, of end pieces of crude magnets, e.g., compression molded or isostatically pressed parts or blocks, parts that have been improperly coated or are useless either magnetically or because of their dimensions as well as excess quantities. This magnetic scrap metal has a relatively high metal value. However, reusing it for production of magnets poses problems and/or is expensive because in this state this material contains impurities, e.g., Ni, C, O which interfere with recycling. Current recycling options consist of using the magnetic scrap material in a new melt, where it is cut with newly weighed-in material. Furthermore it is possible to grind the magnetic scrap metal, remove most of the Ni impurities and process it in a mixture with another newly produced powder having a suitable composition to form sintered magnets. Ultimately, regeneration by direct reduction with calcium is also known. These recycling methods for production of new sintered magnets result in sacrifices in terms of the magnet quality or in high costs. Because of these problems in recycling, large quantities of magnetic scrap metal have already accumulated.
For the production of plastic-bonded magnets, impurities due to the use of magnetic metal scrap would be virtually irrelevant because they would constitute only an insignificant dilution based on their volume. However, if the magnetic scrap metal is to be bonded and the powder is to be processed to bonded magnets, then there is the problem that the coercitive field strength (Hc) decreases greatly during milling if the material does not already have an Hc deficiency. Due to the placement of the magnetic powder in air, the surface and thus the magnetic properties are further damaged by nucleation. Consequently such magnets would not be stable even when used at moderate temperatures or with weak opposing fields.
To produce high-quality anisotropically-bonded magnets based on Nd—Fe—B, German Patent DE 199 50 835 A1 (Aichi Steel) has disclosed a version of the so-called HDDR method. In this method, powder with a good anisotropy and coercitive field strength is manufactured from a lumpy Nd—Fe—B melt having an isotropic distribution of the c axes of the hard magnetic crystals by hydrogenation and dehydrogenation in a special process. For this process, a homogeneous melt which may contain hardly any α-Fe and free Nd must thus be used. In addition, a material with coarse columnar crystals should be used. This method is thus extremely complex and expensive as a result.
As shown in FIG. 2 which illustrates the crystallographic orientation of crystals in the HDDR process, problems occur due to the use of a cast block of an alloy based on NdFeB as the starting material. As shown by the figure on the left, a grain of a parent alloy which corresponds to a crystal has a crystallographic orientation of the c-axis. This orientation is usually different from the orientations of neighboring grains, i.e., there is a random distribution of the orientation of the c axes. The grains in the melt are also relatively coarse. In addition there is the problem of inhomogeneity due to coarse α-Fe and Nd-rich precipitation and deposition.
In the reverse phase transition, which is diagrammed in the middle drawing and/or the drawing on the right, a mixture of RH2, Fe and Fe2B is formed from R2Fe14B, where R stands for a rare earth element. The reactions are explained by the fact that the crystallographic orientation of the c-axis of the Fe2B phase does not change, i.e., the orientation of Fe2B matches that of the grain of the parent alloy. Ultimately, a recombined microstructure is obtained, where the arrows denote the crystallographic orientation of the c-axis of the R2Fe14BHX phase. This phase orientation again corresponds to that of the phase of the parent alloy of the grain.
A similar process for production of anisotropic R-T-B magnetic powder which is also based on hydrogenation and dehydrogenation (HDDR) of fused alloys and its use for bonded magnets are described in German Patent DE 693 15 807.
One problem in the technical implementation of the HDDR method according to the state of the art is the influence of numerous parameters such as temperature, hydrogen pressure, etc., on the one hand, but on the other hand the composition and microstructure of the starting material (melt) also play an important role. This is expressed in a different anisotropy of the resulting melt, which may be manifested, e.g., as the ratio of remanence and saturation polarization, for example. A ratio approaching one is desired but is never achieved in practice.